Herbicides are the most widely used class of pesticides worldwide and are used in agriculture, households and industry. Exposure to two of the most popular herbicides is associated with poorer brain function in teenagers, according to a study led by researchers at the University of California, San Diego (UCSD) Herbert Wertheim School of Public Health and Human Longevity Sciences.
In the Oct. 11, 2023, online issue of Environmental Health Perspectives, researchers report measuring metabolite concentrations of two commonly used herbicides—glyphosate and 2,4-dichlorophenoxyacetic acid (2,4-D)—as well as the insect repellent DEET. Urine samples were collected in 2016 from 519 adolescents aged 11 to 17 years living in the agricultural county of Pedro Moncayo, Ecuador. The researchers also assessed neurobehavioral performance in five areas: attention and inhibitory control, memory and learning, language, visuospatial processing, and social perception.
Researchers at the Herbert Wertheim Institute for Public Health and Human Longevity Sciences at UC San Diego measured concentrations of the commonly used herbicide and repellent DEET in urine samples from adolescents in the agricultural county of Pedro Moncayo, Ecuador, to assess their effects on the adolescent brains. Image credit: Briana Chronister, University of California, San Diego
Main findings and historical background
"Many chronic diseases and mental health disorders have increased globally in adolescents and young adults over the past two decades, and exposure to neurotoxic pollutants in the environment may explain part of this increase," said senior author Jose Ricardo Suarez, MD, PhD. Ph.D., M.P.H., associate professor at the Herbert Wertheim School of Public Health.
Research findings include:
Glyphosate, a non-selective herbicide used for vegetation control in many crops, including corn and soybeans, as well as in residential settings, was detected in 98% of participants.
66% of participants detected 2,4-D, a broadleaf herbicide used on lawns, aquatic sites and crops.
Higher levels of 2,4-D in urine are associated with lower neurobehavioral performance in areas such as attention and inhibitory control, memory and learning, and language.
Urinary glyphosate concentrations were only associated with lower social perception scores, whereas DEET metabolites were not associated with neurobehavioral performance.
Following the introduction of genetically modified glyphosate-resistant “Roundup Ready” crops in 1996 and 2,4-D-resistant crops in 2014, use of glyphosate and 2,4-D increased significantly, making them the most widely used herbicides in the world, the authors write.
Scholars’ concerns and future research
Jose Ricardo Suarez, MD, PhD, MPH, is an associate professor in the Herbert Wertheim School of Public Health and Human Longevity Sciences at the University of California, San Diego. Image source: University of California, San Diego
"The heavy use of herbicides and pesticides in agricultural production in both developed and developing countries around the world increases the likelihood that children and adults will be exposed to herbicides and pesticides, especially if they live in agricultural areas, but we don't know how it affects each stage," said first author Briana Chronister, a doctoral student in the UC San Diego-San Diego State University Public Health Doctoral Program.
Previous research has shown that exposure to some of the most commonly used pesticides alters neurocognitive function, while others may also affect mood and brain development. Today, 20% of teenagers and 26% of young adults suffer from a diagnosable mental health condition, such as anxiety, depression, impulsivity, aggression or a learning disability.
The authors reported that 2,4-D was negatively associated with performance in all five neurobehavioral domains, but statistically significant associations were observed in attention and inhibitory control, memory and learning, and language. Glyphosate only had a significant negative correlation with social perception (a test measuring the ability to recognize emotions), whereas the DEET metabolite was not associated with neurobehavioral changes.
Briana Chronister is a doctoral student in the UC San Diego-San Diego State University Joint Doctoral Program in Public Health. Photo Credit: Brianna Chronister
"Hundreds of new chemicals are launched on the market every year, and more than 80,000 chemicals are currently registered for use," Suarez said. "Unfortunately, very little is known about the safety and long-term effects of most of these chemicals in humans. More research is needed to truly understand the effects."
This study is part of ESPINA: Secondary Exposure to Pesticides in Children and Adolescents, a prospective cohort study funded by the National Institute of Environmental Health Sciences, part of the National Institutes of Health, the National Institute for Occupational Safety and Health, and other private funding sources. ESPINA aims to understand the effects of pesticide exposure on human development from childhood to adulthood.
In 2022, Suarez and his team complete the 14th year of follow-up of study participants and plan to evaluate whether the observed associations persist into early adulthood.